1. Field Of the Invention
The present invention relates to boiling water nuclear reactors having a core shroud disposed concentrically within a reactor vessel and a plurality of tie rods installed in tension between vertically spaced sites on the core shroud at a respective plurality of angularly spaced locations about the periphery of the core shroud to axially compress the core shroud as part of a core shroud repair. In particular, the present invention relates to a method and apparatus for measuring the preload on such tie rods after installation in order to detect changes in the axially compressive preload applied to the core shroud by the tie rods.
2. Discussion of the Related Art
Boiling water reactor shrouds are disposed concentrically within reactor vessels to divide the flow of cooling water through the reactor vessels and to structurally support and align the fuel assemblies, steam separator assemblies and control rod guide tubes. The shrouds are generally cylindrical and are typically formed of multiple arcuate steel plates joined by welds along abutting vertical and horizontal edges. After periods of use, cracking of the shroud within heat affected zones of the welds tends to occur as a result of corrosion, radiation and stress. Cracking of the vertically oriented welds is expected to be minor and is considered acceptable because these welds are relatively short in length, relative to the overall shroud length, and because cracking along the vertically oriented welds does not adversely affect the function of the shroud. When excessive cracking of the horizontally oriented welds occurs, however, the shroud must either be replaced or repaired.
U.S. Pat. No. 5,402,570 to Weems et al, the disclosure of which is incorporated herein by reference, describes a method of repairing boiling water reactor core shrouds having horizontal cracks in heat affected zones of welds by securing plural tie rods in vertical orientation about the periphery of the cracked shroud to axially compress the shroud and thereby urge the opposing surfaces of the horizontal cracks toward one another. The method involves securing the tie rods between sites on the core shroud adjacent top and bottom portions of the shroud. Typically, the upper end of each tie rod is threaded and is made to pass through connection hardware, such as a bracket or beam, depending from the top of the shroud. A nut is then threaded onto the upper end of the tie rod and the tie rod tensioned to achieve a desired tie rod preload corresponding to an acceptable level of compression on the shroud. Tensioning of the tie rod is usually accomplished by threading a puller bar onto the upper end of the tie rod and using a hydraulic cylinder to lift the puller bar upwardly in a vertical direction. The nut is then tightened against the bracket so that, when the puller bar is released, an upwardly directed tensile force is exerted on the tie rod by the nut, and a corresponding downwardly directed compressive force is exerted by the nut on the bracket, thereby holding the shroud in axial compression along its length. This axial compression drives the opposing surfaces of any horizontal cracks toward one another, thereby urging the cracks together across all or part of the thickness of the shroud and preventing their adverse effects on the shroud structure. Axial compression of the shroud also improves the ability of the shroud to withstand axial or tensile loads caused by hydrostatic pressures and/or seismic events since such tensile loads are offset by the compressive preload and are thereby mitigated.
It is desirable for an axially compressive preload to be applied continuously to the shroud by the tie rods and for such a preload not to vary significantly from that originally applied by the tie rods at the time of installation; however, preloads can change after installation due, for example, to seismic events and/or thermal transients associated with the start-up and cool-down of the reactor. For example, changes in tie rod preload could occur due to cracking of the core shroud, settling and wear-in of threaded and other mechanical joints, thermal or neutron relaxation of the tie rods, or failure of the tie rods (for example, as a result of cyclic loading of the tie rods caused by thermal transients). To date, however, it has not been practical to detect changes in the axially compressive preload applied to the shroud by the tie rods due to the inaccessibility of the tie rods and the shroud and the highly radioactive and hostile environment within the reactor vessel.